CSAMT Method Research Articles

Interpreting Surface Large-Loop Time-Domain Electromagnetic Data for Deep Mineral Exploration Using 3D Forward Modeling and Inversion

Surface transient electromagnetic (TEM) data with large transmitter loops for deep mineral exploration are often complicated by the non-trivial coupling between extended sources and arbitrarily oriented geological targets. This case study reports a TEM field data set acquired across terranes with strong lateral inhomogeneity, which is responsible for the high inconsistency in TEM data patterns along the survey line, as well as for the negative TEM transients (sign reversal) at some near-central loop stations. 3D forward modeling and inversion, as maturing tools in recent years, offer unique opportunities to extract as much geological information from such data as possible. 3D forward simulations of representative synthetic models found that the phenomenon of sign reversal at some TEM stations is associated with compact conductors enclosed by the transmitter loop and receivers that are in the loop, but off the conductor–a situation that is common in large-loop TEM and can only be explained by 3D models. However, 3D inversion of the field data with a uniform subspace as the initial and reference model fails to converge, another point of evidence that 3D inversions of large-loop TEM data are more likely to be subject to stability issues. Our solution is to warm-start the inversion with the representative model in the forward simulation experiments as the initial model, so the ill-posed 3D inversion can escape from local minima. Finally, the vertical contact structure in our 3D-inversion model is verified by a resistivity cross section of the CSAMT method. Our case study demonstrates the demand and capability of 3D electromagnetic modeling and inversion for high-resolution deep mineral exploration. It also provides an easy-to-follow template for carrying out 3D interpretation for complex geology in practice.

Distortion effects caused by target abnormal bodies in CSAMT exploration

In CSAMT exploration, the using of the artificial sources not only improves the signal-to-noise ratio of the data, but also brings a series of distortion effects, such as shadow and source overprint effects. This paper attempts to introduce a distortion effect caused by the target in the survey area. Although it is often ignored, it always plagues the data interpretation. In CSAMT method, the primary current has determined direction due to the source. When the primary current encounters electrical interfaces, the induced charge will accumulate on it and generate local current, causing local distortion. The anomaly body stretches in the direction of the vertical primary current, and a false anomaly with opposite polarity appears on both sides of the target. If the direction of the primary current is different, the accumulation position of the induced charge is also different, which will result in different shapes of the anomalies in observed data. This paper confirms the existence of the distortion by taking four simple models as examples and explains it from the physical mechanism. On this basis, the paper summarizes the relationship between inversion and distortion. If our code can simulate the distortion effect in the forward, we do not need to remove it before the inversion. Otherwise, it must be removed.

Combination use of gravimetry, CSAMT and SOTEM methods in geophysical prospection of a polymetallic silver-copper deposit: Case studies in the Inner Mongolia, China

Geophysical methods, which use physical properties to reveal the structure of the underground spaces, have been widely applied in geological exploration. However, a single geophysical approach only reflects the distribution of a kind of physical property and therefore has its limitations. To improve the reliability of geophysical methods in actual geological exploration, in this paper, we are trying to combine the use of a variety of geophysical exploration methods, such as gravity, control source audio magnetotelluric method (CSATMT) and electrical source short offset transient electromagnetic method (SOTEM), to a specific metal in Inner Mongolia. Firstly, the gravimetry method is applied to figure out the anomalous area, and then the CSAMT method is adopted to probe the deep electrical distribution. Finally, the SOTEM method is applied for further auxiliary verification. Results show that the underground structure is well revealed by combing the use of these methods. This paper can be used as a reference for the exploration of concealed ore bodies in polymetallic ore deposits. To use a combination of methods to measure. In the geological exploration of a polymetallic silver-copper mine in Inner Mongolia, a variety of geophysical exploration methods such as gravity measurement, controllable source audio magnetotelluric measurement and electrical source short offset transient electromagnetic method were combined, and geological data were combined. The better exploration results provide reference for the layout of the drilling location of the drilling project, and provide assistance for the prospecting breakthrough of the concealed ore body in the polymetallic mining area.

Exploration of buried carbonate aquifers by the inverse and forward modelling of the Controlled Source Audio-Magnetotelluric data

On the selected cases, Gotalovec in the area of Pannonian basin and Baška in the Dinaridic karst area, that are representing a common hydrogeological model in both regions of Croatia, CSAMT data together with data of other geophysical methods (electrical resistivity tomography, electrical sounding and seismic reflection) enabled the definition of a reliable prognostic geological model. The model consists of carbonate aquifer which underlies an impermeable thick package of clastic deposits. There are great variations of the dolomitic aquifer depths in the Gotalovec area due to strong tectonic activity, while in the Baška area depth changes are caused by the layer folding.The CSAMT method provides the most complete data on lithological and structural relationships in cases of hydrogeological targets deeper than 100 m. Based on the presented models we can conclude that the CSAMT method can provide greater exploration depth than electrical resistivity tomography (ERT) and can be considered as a fundamental geophysical method for exploration of buried carbonate aquifers, deeper than 100 m. But, the CSAMT research may demonstrate its advantages only in the case of very dense layout of CSAMT stations (25–50 m), due to the greater sensitivity to noise in relation to resistivity methods. Interpretation of CSAMT data is more complex in relation to resistivity methods, and a forward modelling method sometimes gives better results than an inversion due to possibility of the use of additional data acquired by other geophysical methods (ERT, electrical sounding and seismic reflection). At greater depths, the resolution of all electrical methods including the CSAMT method is significantly reduced, and seismic reflection can be very useful to resolve deeper lithological interfaces.

APPLICATION OF CONTROLLED SOURCE AUDIO MAGNETOTELLURIC (CSAMT) AT GEOTHERMAL

<p class="abstrak">CSAMT or Controlled Source Audio-Magnetotelluric is one of the Geophysics methods to determine the resistivity of rock under earth surface. CSAMT method utilizes artificial stream and injected into the ground, the frequency of artificial sources ranging from 0.1 Hz to 10 kHz, CSAMT data source effect correction is inverted. From the inversion results showed that there is a layer having resistivity values ranged between 2.5 Ω.m – 15 Ω.m, which is interpreted that the layer is clay.</p><p class="abstrak"><span lang="EN-US"><br /></span></p>

Study on the Application of CSAMT in Geothermal Exploration in Jilin Province

Geothermal resource is a kind of clean energy source. There is rich fracture structure in Jilin city of Jilin province and it has a good geothermal geological conditions. Using CSAMT method and selecting the appropriate technical parameters, we can survey controlled thermal structure in the measured area. The collected data is processed and combined with the known geological and hydrological condition. Interpreting the data, base fluctuations and spatial distribution of buried faults can be got. Then, deep groundwater-rich region and the depth of reservoir burial are inferred.

Metodologia exploratória para demilitação de condutos vulcânicos kimberlíticos com o uso do método CSAMT, MG, Brasil

No presente trabalho apresentamos um estudo de caso onde o método CSAMT (audiomagnetotelúrico com fonte controlada) foi aplicado para determinar a assinatura geoelétrica da estrutura geológica do kimberlito Régis, localizado e conhecido no Estado de Minas Gerais, Brasil. Afloramentos desses kimberlitos foram mapeados na década de 1970. Três perfis CSAMT, dois com direções norte-sul e um leste-oeste foram adquiridos ao longo do corpo. As estações CSAMT foram adquiridas com eletrodos e boninas dispostos na configuração clássica em cruz. O intervalo típico entre estações variou de 50 a 100 m ao longo de cada perfil. A faixa de frequência utilizada foi de 10 Hz a 100 kHz, adequadas para investigações rasas (profundidades menores que 500 m). O presente trabalho deu subsídio para a locação de dois furos de sondagens dentro do pipe estudado e permitiu a proposição de um novo modelo geológico para o kimberlito.

1‐D Forward Modeling of the CSAMT Signal Incorporation IP Effect

AbstractThe response of a geological object to an exciting source of electromagnetic (EM) waves in the frequency domain is the integrated one of EM induction and induced polarization. In the forward and inversion modeling of the conventional CSAMT method, telluric resistivity is assumed to be a real number unrelated to frequency. It is, however, actually a complex number associated with frequency because of the induction polarization (IP) effect. The purpose of this work is to promote the study on the integrated response above mentioned, expand the application area of the IP method and enhance the precision of EM exploration. Based on the Dias model, using complex resistivity to replace direct current resistivity which does not incorporate IP effect, we have made forward simulation of a CSAMT field source on a 1‐D layered model to offer the theoretical basis for extracting the IP information in CSAMT signal. The result shows that the response curves of apparent resistivity and phases (including the far field, transitional field, and near field) have obvious abnormal changes due to incorporation of the IP parameter. The peak anomalies of amplitude ratios and phase differences before and after polarization can exhibit IP anomalies in a direct view. These peak values have a continuous correlation with the polarized layer thickness, buried depth and variations of resistivity. It demonstrates that extracting IP information from EM signals in the frequency domain has a bright prospect.

P67 CSAMT調査による箱根大涌谷における浅部比抵抗イメージ(ポスターセッション,日本火山学会2008年秋季大会)

P67 CSAMT調査による箱根大涌谷における浅部比抵抗イメージ(ポスターセッション,日本火山学会2008年秋季大会)

Application of CSAMT Method for Exploring Coal Mine in Fujian Province, Southeastern China

Application of CSAMT Method for Exploring Coal Mine in Fujian Province, Southeastern China

An All-Frequency Resistivity-Depth and Static-Correction Technique for CSAMT Data, with Applications to Mineralised Targets under Glacial Cover (Western Tasmania) and Basalt Cover (Victorian Goldfields)

New algorithms for apparent resistivity, static corrections, and Bostick transforms of CSAMT data allow real-time production of parasections for resistivity versus depth from profiles of CSAMT data at all frequencies. This allows rapid visualisation of 2D and 3D CSAMT model data and comparison with field data. The resulting interactive fitting of parasections of model and field data provides an alternative to formal inversion.The process is used to model the effect of glacial erosion features in CSAMT data from Western Tasmania in order to assess the sensitivity of the CSAMT method for the detection of targets such as steeply-dipping graphitic and mineralised alteration zones. The approach has advantages of real-time capability (with potential to flag data affected by cultural and power transmission line noise), of disriminating between effects of surficial conductive variations such as filled glacial erosion features, and of differentiating between genuine dipping conductors, vertical conductors, and artefacts from near-surface effects, which may generate apparent “vertical” conductive anomalies.A similar approach using CSAMT data from basalt-covered terrain in the Victorian goldfields, combined with 3D EM modelling, was successful in identifying a prospective sulphidic conductor in the Ordovician basement, beneath variably-conductive basalt cover.

An assessment of CSAMT data for location of mineralised targets under glacial cover (Western Tasmania) and basalt cover (Victorian goldfields)

New algorithms for all-frequency apparent resistivity, static corrections and Bostick transforms of CSAMT data allow real-time production of parasections of resistivity vs depth from profiles of CSAMT data. This allows rapid visualisation of 2D and 3D CSAMT model data and comparison with field data. Interactive fitting of parasections of model and field data provides an alternative to formal inversion. The process is used to model the effect of glacial erosion features in CSAMT data from Western Tasmania and assess the sensitivity of the CSAMT method to the detection of targets such as steeply-dipping graphitic and mineralised alteration zones. The approach has advantages of flagging data affected by cultural (power transmission line) noise, provides clear discrimination between effects of surficial conductive variations such as filled glacial erosion features, and discriminates between genuine dipping conductors, vertical conductors, and artefacts from near-surface effects which may generate ``apparent' vertical conductive anomalies. A similar approach using CSAMT data from basalt-covered terrain in the Victorian goldfields, combined with 3D EM modelling, was successful in identifying a prospective sulfidic conductor in the Ordovician basement, beneath variably-conductive basalt cover.

Controlled Source Audiofrequency Magnetotelluric method applied in urban area of Central Osaka, Japan

A detailed study was carried out to find out the most suitable time for electromagnetic sounding in order to apply the CSAMT method in the urban area of Central Osaka. The preliminary data collected during the daytime were highly noise­ contaminated, which couldn't be attenuated even on increasing the sampling time. Because of this reason, all the soundings were obtained during the nighttime when the electromagnetic noises were drastically reduced. The changes in layer resistivities with depth, as obtained from the 1-D inversion of the observed data, correspond to the different reflectors as seen in the seismic section. It is found that the vicinity of Uemachi Fault, in the western part of the study area, is characterised by high conductivity.

Constraining seismic parameters with a controlled-source audio-magnetotelluric method (CSAMT)

SUMMARY Precise seismic two-way traveltime (TWT) determinations are often limited by the knowledge of the overburden thickness and its seismic velocity. Values between 500 and 1800 m s−1 are conceivable, according to the water content. The CSAMT method has been successfully applied to determine the thickness of the Quaternary fluvio-glacial deposits and Tertiary Molasse (interbedded marls and siltstones) overlying Mesozoic carbonates along a 3 km profile through a syncline in the Swiss Jura fold and thrust belt. The homoclinal structure within the syncline is well constrained by a petroleum exploration Vibroseis profile which indicates a SSE-dipping Mesozoic series and a similarly oriented increasing thickness of Molasse and Quaternary. The CSAMT places tight constraints on the apparent resistivities of the three formations: Quaternary—300–1000 ωm; Molasse—5–15 ωm; Mesozoic—100–1000 ωm. Furthermore, modelling of the CSAMT sounding data provides absolute thicknesses for both the Quaternary (80–20 m) and the Molasse (10–210 m) layers, revealing a true dip of the top Mesozoic substratum of 7° to the SSE. The seismic velocities of the Quaternary (weathering layer) and the Molasse can thus be determined at 800 and 2100 m s−1, respectively.

Application of CSAMT Method for Gold-copper Deposits, Chinkuashih Area, Northern Taiwan

Application of CSAMT Method for Gold-copper Deposits, Chinkuashih Area, Northern Taiwan

Time Domain CSMT Method

CSAMT method is widely used in Japan for mining and geothermal exploration. For geothermal applications we need to study the subsurface structure as deep as 2 km or more. This shifts the CSAMT survey to a relatively lower frequency range, where the CSAMT method tends to become less reliable and less efficient. For this reason we are developing a time domain version of CSMT method at the Geological Survey of Japan. Using a controlled source excitation with a period of four seconds the waveforms of electric and magnetic fields are measured. Noisy data are rejected by visual inspection, and only the high quality data are stacked. The resultant wave-forms are Fourier analysed to get the spectra in the frequency range between 1/32 and 8 Hz. This technique not only gives very reliable data of high repeatability, but also makes the measurement time such shorter than CSAMT methods where the time-consuming measurement needs to be repeated for many different excitation frequencies.